A reversible thermosensitive or thermoreversible recording medium has the property that its transmittance (here and in the following discussion we are referring to transmittance with respect to visible light) varies according to its thermal history. That is, it has hysteresis characteristics in the relation between the transmittance and the temperature. It is therefore possible to create a difference of transmittance between a given part of the medium and another part, and therefore to record image or any other information on the medium, by giving a different thermal history to these parts by use of a thermal head, a modulated laser beam, or like selective heating means.
Examples of the structure of the thermoreversible recording medium are disclosed, for example, in Japanese Patent Kokai Publication No. 55-154198.
The thermoreversible recording medium disclosed in this publication comprises a matrix of a polymer such as a polyester or resin, in which an organic substance of low molecular weight such as behenic acid is dispersed.
FIG. 1 shows the hysteresis curve of variation of transmittance with temperature of this conventional thermoreversible recording medium, with transmittance on the vertical axis and temperature on the horizontal axis. We shall now describe the properties of this conventional thermoreversible recording medium with reference to FIG. 1.
Firstly, in the region of room temperature (RT), this conventional thermoreversible recording medium exhibits either transmittance (A) (opaque state) or transmittance (D) (transparent state) as shown in FIG. 1 depending on its thermal history.
If the thermoreversible recording medium is heated above a temperature T.sub.0 to a temperature T.sub.1, its transmittance (A) or (D) changes to (B). Subsequently, when the thermoreversible recording medium is cooled to room temperature, its transmittance (B) changes to (D), and the thermoreversible recording medium then retains a transparent state (D).
Conversely, if a thermoreversible recording medium whose transmittance was (A) or (D) in the region of room temperature is heated above T.sub.0 and T.sub.1 so as to reach or exceed a temperature T.sub.2, its transmittance (A) or (D) changes to (B) and then (C), that is, its transmittance decreases slightly in comparison to the transparent state (D). Subsequently, when the medium is closed to room temperature, its transmittance changes from (C) to (A), and it then retains an opaque state (A).
The following specific examples of the above properties are disclosed in the Japanese Patent Kokai Publication No. 55-154198.
(1) A thermoreversible recording medium comprising a high molecular weight normal-chain copolyester whose principal components are an aromatic dicarboxylic acid and an aliphatic diol together with docosanic acid exhibited stable transparency when it was heated to 72.degree. C. and then cooled. The opaque state of the medium was restored only when it was re-heated to a temperature above 77.degree. C.
(2) A thermoreversible recording medium comprising a copolymer of vinylidene chloride and acrylonitrile together with docosanic acid and a fluoride lubricant to improve fluidity exhibited stable transparent state when it was heated to 63.degree. C. and then cooled. The opaque state of the medium was restored only when it was re-heated to a temperature above 74.degree. C.
(3) A thermoreversible recording medium comprising a copolymer of vinyl chloride and vinyl acetate together with docosanol exhibited stable transparency when it was heated to 68.degree. C. and then cooled. The opaque state of the medium was restored only when it was re-heated to a temperature above 70.degree. C.
(4) A thermoreversible recording medium comprising a polyester and docosanic acid exhibited stable transparency when it was heated to 72.degree. C. and then cooled. The opaque state of the medium was restored only when it was re-heated to a temperature above 77.degree. C.
However, the range of temperature in which the thermoreversible recording medium in the prior art will be in the transparent state, which is required in applications to displays or image forming apparatus is (77-72)=5.degree. C. in the case of the type (1), 11.degree. C. in the case of type (2), 2.degree. C. in the case of type (3), or 5.degree. C. in the case of type (4), and thus it is not more than about 11.degree. C. In a display in which the character portions are transparent (such makes it easier to view), the temperature control of the thermal head or other thermal means is difficult because the range of temperature in which the thermoreversible recording medium is made transparent is narrow. It is therefore difficult to obtain the transparent state stably when the image is repeatedly formed.
Moreover, with the thermoreversible recording medium of the prior art, the contrast between the transparent state and the opaque state was not large enough and improvement has been desired.
Further, Japanese Patent Kokai Publication No. 57-82088 discloses:
(a) a thermoreversible optical recording medium having a similar composition to the above media, and containing also carbon black which absorbs laser light to generate heat, and:
(b) a thermoreversible optical recording medium comprising a heat generating layer containing carbon black which absorbs laser light to generate heat, and a recording layer having a similar composition to the above recording materials deposited on said heat generating layer.
The above publication also gives two recording methods using this thermoreversible optical recording medium, namely opaque recording and transparent recording. We shall here briefly describe these recording methods with reference to FIG. 1, FIG. 2A, and FIG. 2B. FIG. 2A is a drawing for the purpose of explaining the opaque recording method, and FIG. 2B a drawing for the purpose of explaining the transparent method. Both drawings show partial plan views and sections of the thermoreversible optical recording medium.
(a) Firstly, the opaque recording procedure begins with the recording layer in a completely transparent state. If the layer is not transparent, it is made transparent by heating to a temperature between T.sub.1 and T.sub.2 in FIG. 1, and then cooling to room temperature. Subsequently, as shown in FIG. 2A, areas 13a (only one of them being shown) of heat generating layer 13 corresponding to areas 11a of recording layer 11 at which it is desired to write or record, are irradiated by a small spot laser such that the temperature of written areas 11a rises above T.sub.2 in FIG. 1. This causes only written areas 11a to become opaque, and recording takes place. To erase this recording, areas 13a of the heat generating layer corresponding to said opaque areas are irradiated by a laser with a larger spot and lower energy than that used to form the opaque areas. This irradiation causes the temperature of the opaque areas of recording layer 11 to rise to between T.sub.1 and T.sub.2 in FIG. 1, and the opaque areas therefore return to the transparent state.
The reason why the laser spot used for erasure is larger than that used for recording is that it is difficult to re-irradiate only the opaque areas with the laser beam.
(b) Conversely, in the transparent recording method, the recording layer is initially in an opaque state throughout its surface. If the layer is not opaque, it is made opaque by heating to a temperature above T.sub.2 in FIG. 1, and then cooling to room temperature. Subsequently, areas 13a (only one of them being shown) of heat generating layer 13 corresponding to areas 11a of recording layer 11, are irradiated by a small spot laser such that the temperature of areas 11a rises to between T.sub.1 and T.sub.2 in FIG. 1. This causes only written areas 11a to become transparent, and recording takes place. To erase this recording, the areas of the heat generating layer corresponding to said transparent areas of the recording layer are irradiated by a laser with a larger spot and higher energy than that used to form the transparent area. This irradiation causes the temperature of the transparent areas to rise above T.sub.2 in FIG. 1, and the transparent areas therefore return to the opaque state.
The thermoreversible optical recording medium of the prior art became opaque when it was heated to a temperature above T.sub.2 and cooled to room temperature, and became transparent when it was heated to a temperature between T.sub.1 and T.sub.2, and cooled to room temperature. The following problems were therefore inherent in the opaque recording method and transparent recording method, respectively.
(a) In the opaque recording method, when the opaque area (recording area) was made transparent, it was very difficult to re-irradiate only the opaque area with the laser, and so a larger area which included the opaque area had to be irradiated by a laser with a larger spot. However, as the area surrounding the opaque area was transparent, the transparent area passed more light, the corresponding part of the heat generating layer easily generates heat, and its temperature rose higher than that of the part corresponding to the opaque area. As a result, if the laser irradiation conditions were adjusted so that the temperature of the opaque area of the recording layer was between T.sub.1 and T.sub.2, the temperature of the surrounding area rose above T.sub.2. While the opaque area could therefore be returned to the transparent state, the surrounding area became opaque. If on the other hand the laser irradiation conditions were adjusted so that the temperature of the surrounding area did not reach T.sub.2, the temperature of the opaque area did not reach T.sub.1 and the opaque area could not be returned to the transparent state. In either case, therefore, it was impossible to erase the recording completely.
(b) In the transparent recording method, higher recording densities are achieved if the laser spot which is used for recording is smaller. However, to form a transparent area with such a small spot, the temperature of an extremely minute area of the thermosensitive layer has to adjusted to within a very narrow range T.sub.1 -T.sub.2 which is only of the order of 2.degree.-10.degree. C. or so. Such fine temperature control is very difficult to perform.
Further, an example of the thermoreversible display medium comprising a recording layer of the above recording materials on a colored support member, is disclosed for example in Japanese Patent Kokai Publication No. 62-257883.
In the thermoreversible display medium of this publication, the colored support is black or red with a surface smoothness of no less than 300 sec. Further, the recording layer of this thermoreversible display medium exhibits the same temperature - transmittance variation properties as those of FIG. 1, and image recording and erasure can therefore be achieved by the following method (a) or (b):
(a) The thermoreversible display medium is prepared by heat drying at a temperature of 68.degree. C. The recording layer then becomes transparent and makes the color of the medium the same as that of the colored support, i.e. black (or red), Next, printing is performed on the medium by for example a thermal head heated to a temperature of 76.degree. C. or above. This makes the printed area opaque with white color so that the colored support is no longer visible. An image is thus obtained consisting of white printed areas on a black (red) background.
(b) Conversely to the method in (a), the thermoreversible display medium is prepared by heat drying at a temperature of 76.degree. C. or above. This makes the recording layer white, so the medium looks white. Next, writing is performed on the medium by a heat pen heated to a temperature of 68.degree. C. This makes the areas which were written upon (printed area) transparent so that the colored support is visible only through these areas. An image is thus obtained consisting of black (red) printed areas on a white background.
An example of an image recording device comprising a display medium based on a material whose transparency varies according to its thermal history, and an erasure means to erase the image formed on this display medium, is disclosed for example in Japanese Patent Kokai Publication No. 57-92370 and Japanese Patent Kokai Publication No. 57-89992.
In the image recording device disclosed in Japanese Patent Kokai Publication No. 57-92370, the display medium comprises a recording layer formed from a material having the same temperature - transmittance variation properties as those of FIG. 1. The recording means comprises a writing instrument with a heat head for recording, and the erasure means comprises an erasing instrument with a heat sliding surface.
In this device, an image is formed when a person holding the writing instrument brings its heat head into contact with the display medium, and the image is erased when the heat sliding surface of the erasing instrument is brought into contact with the image. If this device is used to form an image by the opaque recording method, the temperature of the writing instrument is set at T.sub.2 or above, and the temperature of the erasing instrument is set in the range T.sub.0 -T.sub.1. If on the other hand, an image is formed by the transparent recording method, the temperature of the writing instrument is set in the range T.sub.0 -T.sub.1, and the temperature of the erasing instrument is set at T.sub.2 or above.
In the image recording device disclosed in Japanese Patent Kokai Publication No. 57-89992, the display medium comprises a recording layer formed from a material having the same temperature - transmittance variation properties as those of FIG. 1. The recording means comprises a head consisting of a plurality of resistive heating elements, and the erasure means comprises a fluid bath whose temperature can be controlled. The display medium is in the form of an endless loop, and it is advanced by a drive means such as rollers through a certain area including the recording section and erasure section. In this device, an image is formed when the head consisting of a plurality of resistive heating elements comes into contact with the display medium, and and the image is erased when the display medium is immersed in the fluid bath. More specifically, this publication describes an example of image formation by the transparent recording method. In this case, the temperature of the recording means is set within the range 65.degree.-70.degree. C., and the temperature of the fluid bath is set at 80.degree. C. or above.
However, conventional thermoreversible display media (including the display medium used in the above conventional image recording device) have the property that when they are heated to a temperature T.sub.2 or above and then cooled, they become white, while if they are heated to a temperature in the range T.sub.1 -T.sub.2 and then cooled, they become transparent. Moreover, the temperature range T.sub.1 -T.sub.2 required to obtain transparency was no more than 2.degree.-10.degree. C. or so. To form an image on this thermoreversible display medium by the transparent recording method, it was therefore necessary to control the temperature of the recording means consisting of said writing instrument or head to within 2.degree.-10.degree. C. or so of the specified temperature. The writing instrument, head or other part used for printing is however extremely small, and it is very difficult to control the temperature of such a small part precisely.
In the opaque recording method, on the other hand, the conventional display medium becomes opaque at a temperature T.sub.2 and above, and as this temperature range is very large, the problem of controlling the temperature of the recording means is avoided. In this case, however, white printed areas appear on a transparent background, or white printed areas appear against a background which has the color of the colored support. If the contrast between the background and the printed areas is low, therefore, the display is very difficult to see. If the color density of the colored support was increased to improve the quality of the display, it caused eye fatigue because the area of the background is greater than that of the printed areas; while if, on the other hand, the color density of the colored support was decreased, the contrast declined. In either case, therefore, the opaque recording method was not a desirable recording method.
Use of the above-described thermoreversible recording medium in an image forming device utilizing electrophotography has been proposed.
The proposed device charges the surface of a photosensitive member, thermally writes on a thermoreversible recording medium, forms image and non-image portions depending on the difference in transmittance, and performs whole-surface exposure on the photosensitive member, with the thermoreversible recording medium superimposed thereon, to form an electrostatic latent image on the surface of the photosensitive drum.
Developing the electrostatic latent image and transferring to and fixing on the resultant toner image on recording medium, recording is made on ordinary paper.
FIG. 3A to FIG. 3F show the processes of image formation in the above image forming apparatus. FIG. 3A shows the thermal writing process, FIG. 3B shows the charging process, FIG. 3 shows the whole-surface exposure process, FIG. 3D shows the development process, FIG. 3E shows the transfer process, and FIG. 3F shows the fixing process.
In the above-described image forming processes, thermal writing is first conducted on a thermoreversible recording medium 23 moving over a platen roller 22 using heat-emitting elements 21. As a result, an image represented by differences in density or transmittance is formed on the thermoreversible recording medium 23. That is, the thermoreversible recording medium 23, the entirety of which initially assumed the opaque state as indicated by hatching, now have image portions 24 (unhatched portions) into which thermal writing has been conducted, and non-image portions 25 (hatched portions) into which thermal writing has not been conducted and which assume the opaque state (FIG. 3A).
The photosensitive member 26 is uniformly charged by means of a charging means, i.e., a corona charger 27 (FIG. 3B). In the illustrated example, a positive-type photosensitive material is employed, and positive charges are accumulated on the surface of the photosensitive member 26. The photosensitive member 26 is formed of a conductive support 26a and a photoconductive layer 26b formed over the conductive support 26a.
Next, the thermoreversible recording medium 23 is superimposed on the photosensitive member 26, which is then subjected to whole-surface exposure through the thermoreversible recording medium 23 by means of a whole-surface exposure means 28. Then, the photosensitive member 26 is irradiated with light in an amount dependent on the image represented by the differences in the density or transmittance. In the illustrated example, the image portions 24 (unhatched portions) are transparent, so light passes therethrough to irradiate the photosensitive member 26 and to remove the charges from the photosensitive member 26. The non-image portions (hatched portions) are opaque, so amount of light which passes therethrough is limited and the charges on the photosensitive member 26 are retained. As a result, the electrostatic latent image on the photosensitive member 26 is formed (FIG. 3C).
In the developing process (FIG. 3D), electric lines of forces are created in the space between the developing roller 29 and the photosensitive member 26, due to the electrostatic latent image. The charged toner 30 on the developing roller 29 is attracted to the photosensitive member 26, moves along the electric lines of force and is attached to the photosensitive member 26. Thus, a toner image is formed on the photosensitive member 26. In the illustrated example, reversal development is performed.
In the transfer process (FIG. 3E), a recording medium 31 is superimposed on the photosensitive member 26, and the toner image on the photosensitive member 26 is electrostatically transferred to the recording member 31 by means of a corona charger 32.
In the fixing process (FIG. 3F), the toner image on the recording medium 31 is heated and melted by a fixing means 33, i.e., a heating roller 34 and a fixing roller 35. The molten toner 30 permeates the fibers of the recording medium 31 and is fixed by application of pressure.
In the image forming apparatus of the above configuration, the range of temperature in which the thermoreversible recording medium 23 is made transparent is narrow, so it is difficult to regulate the temperature within the above range even through control of the current value and the resistance of the thermal head, and obtain constant transmittance when the image forming is repeated.
Moreover, the transmittance is determined by the ratio of the matrix component and the organic substance of low molecular weight, and when the content of the organic substance of low molecular weight is high the transmittance in the transparent state is low, while when the content of the organic substance of low molecular weight is low the density in the opaque state is low, so a sufficient contrast is not obtained.
Moreover, when the prior-art thermoreversible recording medium 23 was used, it is necessary to control the heat-emitting recording elements to maintain the thermoreversible recording medium 23 within the narrow range of from T.sub.1 to T.sub.2, and such control is difficult.